In a compound,the constituent elements are always present in a definite proportion by:

Hint: A chemical compound is a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds.

Complete step by step answer:
In a compound constituent elements are always present in a definite proportion by fixed ratio.
Two or more elements combine to form compounds. A compound is a substance formed when two or more chemical elements are chemically bonded together. Two types of chemical bonds common in compounds are covalent and ionic bonds. The elements combine together in fixed ratio e.g. in water, hydrogen and oxygen elements are present in the ratio of 2:1.

This is proved by the law of conservation of matter. It states that matter is neither lost or gained during a chemical reaction.
This is also proved by the law of constant proportion by Proust.
Despite being a building block in the development of chemistry, the law of constant proportions does not hold true for all chemical compounds. Some exceptions to this law are listed below.
-Some non-stoichiometric compounds have varying compositions of elements between samples. These compounds obey the law of multiple proportions instead.
-One such example is wustite, an oxide of iron with the chemical formula FeO. The ratio of iron and oxygen atoms can range from 0.83:1 to 0.95:1.
-This is caused by the crystallographic vacancies in the samples caused by a disorderly arrangement of atoms.
-Various samples of a compound may vary in the isotopic composition of its constituent elements. This can lead to fluctuations in the mass ratios.
-The differences in the mass ratios between samples are very useful in the process of geochemical dating, due to the preferential concentration of isotopes in many deep Earth and crustal processes.
-This also occurs in many oceanic, atmospheric and even astronomical processes. Despite the effects being quite small, the challenges in the measurement of the effects have been overcome by modern instrumentation.
-Since natural polymers can vary in their compositions, various samples can show different mass proportions.
The answer is a fixed ratio.


Note: The chemical compounds always combine in specific fixed weights. It is not possible to make a compound where there is no definite proportion in between the molecules.